1. Field of the Invention
This invention relates to protective and encapsulant coating materials characteristics for semiconductor elements.
2. Description of the Prior Art
Heretofore, some prior art methods provide coating at least preselected exposed surface areas of semiconductor elements with electrically insulating oxide materials. Such coatings are then layers and have virtually no resistance to mechanical abrasion and require relatively expensive processing equipment. In almost all instances, a second and a thicker coat of a protective coating material is provided to protect the initial electrically-insulating material. Silicon greases, varnishes, rubber and resins which are employed as the overcoating of protective material have been found lacking in desirable physical characteristics.
Robert R. Shaw in U.S. Pat. No. 3,615,913, granted on Oct. 26, 1970, teaches the employment of a coating of a cured, protective coating material selected from the group consisting of polyimides, and polyamide-polyimides disposed on exposed end portions of at least one P-N junction to provide passivation thereof. Although these materials exhibited good abrasion-resistance properties, the passivation requirements of the semiconductor element still require improvements to be made thereto.
Additionally, Shaw made use of silicon oxide, glass fibers, boron nitride, aluminum oxide, quartz, mixa, magnesium oxide and reactivated polytetrafluorethylene and the like to control the consistency of the polymeric material for application to selected surface areas. Alizarin has also been admixed into various coating materials to aid in a sort of surface cleanup treatment for semiconductor materials.
There is currently wide-spread use of oxide/glass layers for passivation and encapsulation of semiconductor devices where device stability and long life are important considerations. However, if the glassy layer must be applied after aluminum metallization, (a wide-spread requirement), the choice of suitable glass systems is severely circumscribed by a maximum permissible application temperature of .about.577.degree. C. This restriction is set by the aluminum-silicon eutectic and must be carefully observed in all processing operations following aluminization of the silicon.
Several glass coating methods are currently in use. These include chemical vapor deposition (CVD), glass frits, and spin-on glass forming alcoholates. The last method is only capable of forming very thin layers, of the order of 2000 A, of glasses which tend to be more reactive than desirable and, therefore, are of restricted utility in packaging. Glass frits are widely used in packaging but are not usually employed for surface passivation because of difficulties in formulating glasses with an adequate expansion match to silicon, and which are at the same time suitable passivants and chemically stable. CVD methods permit adequate thickness, a wide choice of composition, expansion matching, etc. but difficulties in controlling sodium contamination in CVD reactors have made it difficult to obtain acceptable passivation layers by direct deposition onto base silicon. This method is, therefore, usually restricted to use as an overcoating of SiO.sub.2 and metallization layers. None of these methods in their current state of development is considered capable of providing a reliable passivation/encapsulation method for large thyristors and other power semiconductor devices.
Recently, a passivation coating material was developed for coating electronic components. The material is a copolymer which is a reaction product of a silicon-free organic diamine, an organic tetracarboxylic dianhydride and a polysiloxane. This material is a significant improvement over prior art materials, exhibiting better adhesion and corona resistance than available polyimide and polyamide-imide coatings. In view of its useful surface characteristics, it is desirable to use such material for coating high voltage semiconductor devices. In such devices it is desirable that the surface coating reduce the high electric fields which occur at the silicon surface to low enough values so that surface breakdown or corona does not occur in the surrounding air, and surface leakage is not significant. It is difficult to do this with very thin layers of polymer materials. The building up of thick layers of defect-free coatings is time-consuming and expensive.
It is an object of this invention to provide a new and improved material for use as a passivation coating material for electronic components which incorporate selective filler materials to enhance the electronic characteristics thereof.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.